On Fri, 05 Aug 2005 17:29:48 GMT, Ignoramus19023
wrote:
On Fri, 05 Aug 2005 17:16:59 GMT, Martin Whybrow wrote:
"Ignoramus19023" wrote in message
.. .
I am considering adding some run capacitors to my self starting RPC. I
am reading Jim Hanrahan's article at
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
and I am confused by something. I understand how self starting
RPCwould start with one cap between one leg 1 and leg 3 (the generated
one). That's how mine is wired. Jim makes a point that it works, but
makes unbalanced voltage.
But why would it start is capacitors are connected between 1-3 AND
2-3, like in this pictu
http://www.metalwebnews.com/howto/ph-conv/fig1.html
I cannot see how it would create assymmetric fields needed to spin up
the motor. Is that because capacitances across legs 1-3 are greater
than capacitance between leg 2-3?
I could try to use run caps at run time and start caps at start
time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
could set the RPC to start on start cap (both caps between leg 1-3)
and then reconnect the same caps to become run caps, one between 1-3
and another between 2-3. Same TDR could, then, turn out output current
aftet time delay, allowing the RPC to spin up and switch to the run
mode.
As you can see, I am quite confused, but am willing to experiment. I
have 4 unused Furnas 75 A contactors that I can wire, with the time
delay relay, to do just about anything.
Idler: 10 HP
Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
use 2 for the starting leg, so three are unused.
i
You are correct saying that it's because the capacitance between 1 + 3 is
higher than between 2 + 3.
My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 + 3 and
500uF switched by a start circuit between 1 + 3. The 500uF is a bit too
much, it starts very quickly (around 1/3 second) so I could probably drop
that to around 200uF.
Thank you Martin.
I have five 92 mF caps total.
What I have now is 2 capacitors connected between leg 1 and 3.
It seems that about the only thing that I need, then, wire it as follows:
1. 1 cap (92 mF) between legs 1 and 2, permanently connected.
2. 1 cap (92 mF) between 1 and 3, permanently connected.
3. 2-3 more caps (180-270 mF) between legs 1 and 3, connected
at start time and switched off by the time delay relay (TDR),
say 4 seconds after startup.
4. (optional) add another contactor on output side that would only
switch output on when the TDR actuates and switches off the start
caps.
Theat would get me an RPC that is:
- properly balanced across the range of output HP
- starts quickly
- only turns on when balanced power is output and full rotational
speed is achieved, that is, when good quality 3 phase power is available.
Does it make sense?
i
With five equal value oil filled capacitors your options are fairly
limited but you can still finish up wth a perfectly good converter.
One thing to remember is that capacitors of this type have extremely
low internal series resistance (milliohms)- that's why you get a
sizable bang if you short circuit a charged one!
The same thing happens when you connect a charged capacitor to an
uncharged one - VERY high peak currents flow as the charge voltage
equalises on the two capacitors.
The capacitors are not greatly bothered by this treatment but it's
very unkind to the switch contacts or relay contacts used to parallel
connect two capacitors if there is a substantial voltage difference at
the instant of connection.
Brute force oversizing of the switching contacts can give reasonable
contact life but it's much neater to avoid the problem by using
separate start and run capacitors that are never parallel connected.
This is easily done with your capacitor collection.
Use three parallel connected as your start capacitor. Only in circuit
for the few seconds needed for the idler to run up to speed.
You now have a bit unbalanced but perfectly usable converter system.
Ideally the load motor should not be switched in until the idler is up
to speed. However, if the load motor is initially running light, this
start capacitance is probably enough for a simultaneous idler and load
start.
Your START switching should be a changeover contact which EITHER
connects the start capacitor OR the run capacitor across L1 and L2.
For the run capacitor, try one capacitor or two capacitors series
connected to halve the effective value - whichever gives the best
voltage balance with full load on the load motor.
Do not place a capacitor across L2 L3. With the values you have
available this would do more harm than good.
Jim